1. Field of the Invention
The present invention is related to a microphone system, a sound input apparatus, and a method for manufacturing the microphone system and the sound input apparatus.
2. Description of the Related Art
When telephone communications established by telephones, speech recognition, voice recording operations, and the like are carried out, it is desirable to collect only target voices (voices of users). However, under use environments of sound input apparatuses, sounds other than target voices such as background noise may be present. As a result, sound input apparatuses having functions capable of eliminating noises have been actively developed, as disclosed in JP-A-7322388.
As technical ideas capable of eliminating noises under use environments where these noises are present, methods for eliminating noises by applying sharp directivity to microphones have been proposed in the technical field.
Also, in recent year, compactnesses of electronic appliances have been progressed, so that such technical ideas capable of making sound input apparatuses compact may constitute important factors.
As microphones having directivity, a differential microphone is known which produces a difference signal indicative of a difference between voltage signals outputted from two sets of microphones, and then, the produced differential signal is utilized. FIG. 13A and FIG. 13B are diagrams for schematically indicating an area which shows a directional characteristic of a normal single microphone, and another area which represents a directional characteristic of a differential microphone. Sound pressure levels (sensitivities of microphones) with respect to a direction of a sound source have been indicated by distances measured from centers to an outer edge of an area 900, an outer edge of an area 910-1, and an outer edge of an area 910-2.
FIG. 13A is a diagram for showing a directional characteristic obtained in such a case that a single microphone having an omnidirectional characteristic is arranged at a center. The omnidirectional microphone has no directivity, while sound pressure levels (namely, sensitivity of microphone) along respective directions are constant.
FIG. 13B is a diagram for indicating a directional characteristic of a differential microphone constituted by two sets of microphones. A relationship between positions and angles of these two microphones is represented in FIG. 14, namely, a perpendicular direction with respect to a straight line which connects two microphones is defined as 0 degree. The directional characteristic of the differential microphone becomes such a dual directional characteristic that sound pressure levels (sensitivities of microphones) become maximum values along both a direction of 90 degrees and a direction of 270 degrees, whereas sound pressure levels (sensitivities of microphones) become zero along a direction of 0 degree and a direction of 180 degrees.
As a consequence, in order to collect only a target voice by employing a differential microphone, an arrangement of the differential microphone with respect to a sound source constitutes a major factor.